This invention is concerned with improving the speed of combustion in certain localities of a combustion chamber which are now inherently slow. Speed of combustion of a mixture is to be distinguished from the ignitability of the mixture. Ignitability, to a combustion engineer, means momentary oxidation of isolated combustible elements facilitated by an independent heat source. Whereas, speed of combustion, to that same combustion engineer, means the rate at which continuous self-sustained oxidation takes place relying upon self-generated heat to transfer ignition to subsequent combustible elements.
The prior art has done very little to influence or control the speed of combustion in certain selected zones of a four-stroke cycle internal combustion engine, other than to refer to such goal as desirable and best achieved by the right combustible mixture.
As an illustrative example, consider charge stratification in a reciprocating engine in which a relatively lean combustible mixture is inducted into the main combustion chamber with a measured quantity of fuel impregnated into a part of the lean mixture adjacent the spark plug electrodes. The impregnated mixture adjacent the electrodes has excellent ignitability and burns somewhat rapidly until the flame front reaches the lean mixture where it is then slowed down considerably, and possibly extinguished without the ability to be re-ignited depending on the leanness of the mixture. The overall mean combustion velocity for all parts of the main combustion chamber is thus low while initial ignitability is good. Due to the low mean combustion velocity, the combustion stroke is prolonged leading to negative work and diversion of energy to heat losses rather than to positive mechanical work. As a result, fuel economy and engine efficiency become unsatisfactory.
What is needed is a practical means by which any mixture, lean or rich, can be: easily ignited, efficiently combusted to burn with a predetermined and controllable velocity which may be very fast, if desired, and programmed to have ignition or burning in any zone of the combustion chamber. The prior art has attempted in many ways to obtain these goals. One was by inducing swirl in the combustion chamber resulting from changing the chamber shape but did not realize a significant increase in combustion velocity. Others have tried to change piston motion which resulted in only moderate achievement, and still others are working with centralized high energy ignition sources which require compact combustion chambers, the latter results in less than desirable combustion velocities particularly at lean mixtures.
An increase in combustion efficiency depends upon (1) the expansion efficiency of the combusted gases, (2) heat rejection losses, and (3) the type of working fluid. With respect to factor (3) the working fluid is dictated by commercial realities to be gasoline having certain octane ratings; this results in a triatomic mixture of fuel elements and air when it is burned. To optimize combustion velocity through this factor, a stoichiometric mixture is desirable, but fuel economy, engine efficiency, or emissions usually dictate that an ultra-lean mixture be used in at least a part of the combustion chamber. However, the prior art, in turn, cannot cope with excessively lean mixtures because they either cannot be ignited or lead to slow combustion. Thus, the working fluid must remain in accordance with prior art knowledge. With respect to factors (1) and (2), they are influenced herein by redesign and re-arrangement of combustion apparatus to achieve a controlled increase in combustion velocity and thereby provide better engine efficiency, fuel economy and lower emissions. To simultaneously improve or change the expansion efficiency factor and rejection heat losses, a deeply penetrating and controllable torch is generated having a large moving surface which induces entrainment of the surrounding uncombusted mixture and creates a significant rapid interchange or mixing motion between the combusted flame front and combusted mixture without impinging on the walls of the chamber or piston.
The need for controlling the rate of combustion transcends reciprocating engines and is important in a rotary internal combustion engine. The need for controlling the rate of combustion may be even greater in a rotary engine due to the configuration of the main combustion chamber and the dynamics of the gas flow therein. The combustion chamber of the rotary engine is a stretched out volume between the rotor and the rotor housing surfaces. With conventional carbureted spark plug ignition operation, the flame propagation or burning at the trailing end of the combustion chamber is relatively slow. The reason for slow burning is due to (a) the absence of flame-propagating charge motion and (b) the tendency for flame quenching due to small distances between the rotor and the rotor housing surfaces. A high velocity transfer flow of the burning gases takes place from the trailing half of the combustion chamber to the leading half; due to the high rate of mass flow along the large combustion chamber surfaces, a substantial heat transfer takes place from the combustion products to the surfaces. The late and partially quenched combustion at the trailing end causes power losses, fuel consumption increases, and higher than desirable base hydrocarbon emission from the engine.
The common conception that a rotary engine should possess good expansion efficiency by inherently good flow is not entirely true. Expansion efficiency is dependent on good mixing; mixing is a displacement problem and not a flow problem. The continuous rotary flow of gases in a rotary engine does not achieve superior mixing. It is true that local turbulence is created along the flow path of the gases in a rotary engine, but such turbulence (or eddy movement) is not equivalent to a large mass kneading into itself. Rather, such local turbulence can be equated to a vibratory motion of molecules resulting in little net mixing.
Attempts by the prior art to decrease heat rejection losses in a conventional rotary engine by lowering the temperature of combustion through the use of a lean mixture has not resulted in a decrease of fuel consumption. It was hoped that the decrease of the heat rejection losses would result in a more complete combustion process. The inability to achieve a better combustion process at lower combustion temperature is due to the excessively slow burning rate at the trailing end of the main combustion chamber. Thus, more complete combustion is never achieved. The best fuel consumption in a spark plug ignition rotary engine is obtainable with the faster burning rich mixtures. Data to support this indicates that the best economy air/fuel ratio is usually between 13:1 and 14.5:1.
A structural element herein implementing the ability to achieve a deeply penetrating and controlled torch to obtain improved mixing during combustion is the use of a precombustion chamber, sometimes referred to hereinafter as a prechamber. It is important to point out that precombustion chambers have been successfully used for other purposes in diesel and gasoline engines. In diesel engines, the precombustion chamber has been used to improve fuel vaporization by injecting the fuel onto hot surfaces for thereby promoting mixing between the vaporized fuel and the air. In certain reciprocating engines, such as the Honda CVCC, a prechamber has been utilized to improve the ignitability of moderately lean mixtures. However, the operation and function of the precombustion chamber in this invention is not necessarily to only improve ignitability or to improve fuel vaporization, but rather to increase the velocity of combustion in selected zones of the main combustion chamber, particularly in the trailing end portions of the stretched out volume of a rotary engine. The Honda CVCC prechamber has not been able to generate a high energy flame front proceeding substantially through the main chamber and thus has not, nor was it intended to increase combustion speed. Moreover, the prechamber structural element is employed as fixed elements in the sidewall of the rotary engine not in its annular periphery; all prior art attempts to employ a prechamber have either been in a reciprocating engine (differing substantially with respect to the type of technical problems encountered) or in a rotary engine where it has been employed as a moving cavity in the rotor itself or in the rotor housing periphery requiring a short shallow flame front. Whether based both upon a difference in purpose for the prechamber or based upon differences in physical configuration and location of the prechamber, the prior art has been unable to realize the advantages of combustion velocity control as taught herein.